The present invention is generally related to frequency modulated (FM) television systems and, more particularly, to FM television systems capable of carrying television channels without removing amplitude modulated (AM) information from the audio carrier. The present invention is intended for use in FM television systems such as, but not limited to, satellite, MDS, MMDS, CATV cable supertrunk, and fiber supertrunk systems.
In the United States, terrestrial television broadcasts and most cable television (CATV) distribution is made according to National Television Systems Committee (NTSC) standards. FIG. 1 is an amplitude-vs.-frequency diagram illustrating in simplified form the RF spectrum of a typical NTSC television signal, referenced to the lower edge of a channel. NTSC standards require that picture information be separated into two components: luminance, or brightness, and chrominance, or color. The composite television signal 10 of FIG. 1 includes a luminance signal 12 and a chrominance signal 14. A composite television signal is one in which chrominance is carried on a subcarrier. (Other composite signals are SECAM, which is used in France, and PAL which predominates in the rest of Europe). The signal occupies a nominal bandwidth of 6 MHz, with the picture carrier 16 being 1.25 MHz above the lower end of the band. Chrominance information is modulated onto a color subcarrier 18 which is combined with the baseband luminance information. This composite baseband is in turn used to amplitude modulate the picture carrier 16. The color subcarrier 18 has a frequency of 3.579545 MHz, a standard established by NTSC.
In terrestrial television broadcasts and CATV distribution, audio information is frequency modulated onto another carrier 20 lying near the upper edge of the band. The audio carrier 20 has a frequency that is 4.5 MHz greater than that of the video carrier, another standard established by NTSC. The choice of 4.5 MHz for the audio subcarrier frequency represents a tradeoff of signal quality for minimum spectral bandwidth in domestic AM television systems. At this relatively low subcarrier frequency some spectral overlap occurs between video and audio information, and some degradation of the signals is tolerated due to the filters required to later separate the signals.
A television receiver receives both carriers simultaneously and extracts the original composite baseband signal. The composite baseband audio and video signals contain the necessary information to reconstruct the original picture and sound information.
Some systems transmit video and audio by frequency modulating a carrier with the processed composite video-plus-audio-subcarrier waveform. These will be referred to as FM television systems. These systems are often used in satellite television, some CATV supertrunks, studio to transmitter links and other applications requiring high quality video transmission. This process is particularly useful and effective in preserving signal quality through high noise, high loss, low signal level or high interference channels.
The tradeoff discussed above regarding the choice of frequency for the audio carrier can be alleviated in FM television systems. In FM television systems, the occupied bandwidth of the signal is not simply related to the baseband bandwidth. It is possible to use higher frequency subcarriers than 4.5 MHz, and not increase occupied bandwidth. Since FM systems are often used where minimum signal degradation is required, higher frequency subcarriers are often used. To compensate for the increased noise at higher frequencies in FM systems, the deviation of the subcarrier is also increased.
Problems can occur when attempting to carry scrambled video channels on FM television systems. Transmitted video is often scrambled or encoded to prevent viewing by unauthorized or non-paying persons. In many scrambling systems and their variations, the information required to decode or descramble the video is sent as AM on the FM audio carrier. In a similar fashion, many systems transmit terminal control or address information as AM on the audio carrier. At times, amplitude modulation of the audio carrier may be used to predistort the audio carrier in an effort to reduce interfering buzz due to the descrambling process as discussed in commonly assigned U.S. Pat. No. 4,922,532 and entitled "Synch Supression Scrambling And Descrambling Television Signals For Subscription TV" incorporated herein by the foregoing reference thereto. Other applications of AM on the audio carrier exist. In the process of multiplying the audio carrier from the normal 4.5 MHz to a higher frequency, FM television systems destroy AM information on the audio carrier, as described in the following section.
FIG. 2 is a block diagram illustrating a prior art FM video transportation system. Audio may be supplied as baseband and processed internally by baseband processor 2 and FM modulated by FM modulator 3. In this instance, there is no provision for amplitude modulated information on the audio carrier. Rather than the internally modulated audio, switch 5 can select an alternate path. In this alternate path, the audio is supplied as a previously modulated carrier. This carrier is normally frequency modulated with the audio signal, and may also be carrying amplitude modulated information as described above. This carrier is typically multiplied at multiplier circuit 4 by a predetermined rational number N/M and proceeds through the switch 5 to summation circuit 6.
The summed composite video and subcarrier(s) then frequency modulate a carrier at FM modulator 7. The output of FM modulator 7 may then be processed, frequency converted and transmitted as schematically indicated at block 8 in accordance with any of a number of prior art processes through a channel 9.
At the receive end, the signal may be frequency converted and further processed as indicated at block 10 and FM demodulated by FM demodulator 11. It is then separated by filter circuits 12 and 13 into video and subcarriers, respectively. The audio subcarrier can be demodulated by demodulator 14 into baseband. The audio subcarrier can also be multiplied by multiplier 15 with the reciprocal rational number (M/N) to the multiplier 4 in the transmitter and sent to the output as a 4.5 MHz carrier.
The above-described system is unable to pass any amplitude information on the audio carrier. If the audio is modulated internally by FM modulator 3, no provision is made for amplitude modulation. If an external audio subcarrier is brought in with amplitude modulated information, multiplier 4 acts as a limiter and removes the information. Multiplier 15 at the receive end also acts so as to remove amplitude information. Finally, limiting action in the subcarrier FM demodulator 14 will also tend to remove any AM information.
When a system such as that described above is required to pass audio carrier amplitude information, a 4.5 MHz audio carrier is typically summed with the baseband video (or, in one case, a low frequency video carrier) externally, and both are passed through the video baseband circuits 1. At the receive end, a composite video plus subcarrier output can be used that bypasses the internal separation circuits as indicated at 16. Though this technique will work, it combines the video with a 4.5 MHz subcarrier. As previously discussed, this is less than optimal. In most applications, a subsequent separation of the two is required. This separation will be imperfect and thus the scheme conflicts with the goal of low signal degradation, which may have been the original reason for using an FM system.
As discussed above, it is desirable, in an FM television system, to alter the frequency of the audio subcarrier to a frequency higher than 4.5 MHz. Ideally, the audio carrier processing circuits should possess the following properties. First, the frequency of the audio carrier should be increased at the transmitter and restored at the receiver to exactly the original 4.5 MHz. The increase in frequency is required to better separate video and audio information. Exact frequency restoration is required in order to remain within FCC intercarrier frequency tolerances.
Second, the deviation of the audio carrier should be increased at the transmitter and restored at the receiver to exactly the original deviation. The increase in deviation is required to offset the additional noise at the higher subcarrier frequency. Exact restoration to the original deviation is required for BTSC stereo.
Finally, amplitude information on the audio carrier must be preserved in order to pass scrambled video systems.
None of the known prior art accomplishes all three of these objectives simultaneously.